The present invention relates to a process for minimizing the mold deposit problem frequently encountered when non-reinforced polyoxymethylene molding compositions are molded. Such deposits are detrimental to the formation of a quality molded article having smooth and uniform surface characteristics.
As is well known, polyoxymethylene, or polyacetal, is a thermoplastic resin which finds wide utility in the manufacture of shaped articles by injection molding or extrusion processes. Polyoxymethylene has many excellent mechanical properties which result in shaped articles characterized by their hardness, strength and toughness.
Polyoxymethylene resin, however, is subject to degradation particularly under the influence of heat, the amount of degradation being a factor of the method of preparation of the polyoxymethylene and the like. The degradation may occur for example, as the result of oxidative attack. The oxidative attack, which may lead to chain scission and depolymerization, is often retarded by the addition of antioxidants to the polyoxymethylene composition. Degradation is also believed to occur as the result of acidolytic cleavage of the polymer chain caused by acidic species present in the polymer. The acidic species may be acidic catalyst residues derived from catalysts used in the formation of the polymer or may be acetic acid generated from acetate end groups when a given chain, so stabilized, depolymerizes as a result of occasional oxidative or acidolytic chain scission. To assist in minimizing such degradation of polyoxymethylene especially during subsequent processing in the hot, or melt, state, "acid scavengers" are often admixed with the polymer composition.
Albeit most commercially available polyoxymethylene is "prestabilized" either by means of acetylation or hydrolysis treatments (see, for example, U.S. Pat. No. 3,839,267) or by the addition of additives such as the above-mentioned antioxidants and/or acid scavengers, it has been found that during the high-temperature molding of a non-reinforced polyoxymethylene molding composition, particularly, injection molding of the resin an objectionable film, or mold deposit, commonly forms on the surface of the mold. The mold deposit, which can lead to surface defects on the molded resin, is generally believed to be of two types. One type of mold deposit is believed to be caused by the use of certain antioxidants which plate out on the mold surface. This type of mold deposit can be eliminated by using a less volatile antioxidant. The second type of mold deposit is believed to be caused by formaldehyde (generated, for example, as a result of the chain scission of the polyoxymethylene under the conditions of the molding process) condensing on the mold surface. The chain scission, in turn, is believed to be caused by acidic residues present in the polyoxymethylene and which have not been "cleaned up" by the prior stabilization treatments.
Polyoxymethylene compositions thermally stabilized with certain carbodiimides are disclosed in British Pat. No. 993,600. The patent suggests, however, that the primary importance of the carbodiimides is their anti-aging effect and that the carbodiimides have no significance in the stabilization of polyoxymethylene under the usual test methods for thermostability at temperatures of 180.degree. to 220.degree. C. (disclosed to be heating in an open vessel under nitrogen for at least 20 minutes). It is stated that under such conditions the carbodiimides may even increase the decomposition of polyoxymethylenes after about 20 minutes heating time. The patent broadly suggests that mono- and polycarbodiimides, generally, function in the manner stated above, but the specific working examples are limited to the use of severely hindered carbodiimides such as 2,6,2',6'-tetraisopropyldiphenylcarbodiimide and the polycarbodiimide of 1,3,5-triisopropyl-benzene-2,4-diisocyanate.
Carbodiimides are also employed in the acylation of high molecular weight polyoxymethylenes as disclosed in U.S. Pat. No. 3,170,896. The carbodiimides serve to reduce acidolytic degradation during acylation of the polymers caused by traces of free acids present in the organic acid anhydrides employed in the process. Monocarbodiimides and poly-functional carbodiimides are generically disclosed as being useful in the process of the patent although specific examples are limited to mono- and biscarbodiimides. The reason for this is to maximize solubility in the acylating medium. Polycarbodiimides are not desirable because of their more limited solubility. It is also clear that the procedures used indicate a desire to remove any carbodiimides or carbodiimide reaction products after acylation takes place. Following the acylation reaction the acylated polymers are filtered from the reaction medium and freed of reaction medium residues including the carbodiimides by appropriate washing. There is no appreciation of the fact that future processing of the polyoxymethylene could generate acidic species such as formic acid in excess of that which can be normally handled by a standard stabilizer package which ultimately could result in degradation of the polyoxymethylene and mold deposit formation.
U.S. Pat. No. 3,135,718 discloses the polymerization of formaldehyde in the presence of organic acid anhydride acylating agents. Carbodiimides are disclosed for use in purifying the acylating agents of the process of the patent. The class of carbodiimides disclosed to be useful in the process is substantially identical to that of U.S. Pat. No. 3,170,896.
In the past certain polycarbodiimides have been employed as heat and hydrolysis stabilizers for polyesters as well as in a variety of other areas as illustrated in U.S. Pat. Nos. 3,193,522; 3,193,523; 3,296,190; 3,575,931; and 3,835,098; U.S. Ser. No. 715,946, filed Aug. 19, 1976; U.S. Ser. No. 753,384, filed Dec. 22, 1976; British Pat. Nos. 1,056,202; 1,231,975; and 1,330,036; Japanese Document No. 75-00044 (summarized in Chemical Abstract, 172327W, Vol 82, 1975); Belgian Pat. No. 626,176 (summarized in Chemical Abstract, 2054f, Vol. 61, 1964); and Preparation of Carbodiimides from Isocyanates, by W. Neumann and P. Fischer, 1 Angew. Chem. Internat. Edit. 625 (1962).
It is an object of the invention to provide a process for the preparation of an improved non-reinforced polyoxymethylene molding composition, i.e., a stabilized non-reinforced polyoxymethylene molding composition which forms substantially reduced mold deposits upon molding.
It is a further object of the invention to provide a non-reinforced polyoxymethylene molding composition having high stability when subjected to the influence of heat and particularly when subjected to the conditions typically encountered during molding operations.
It is still another object of the present invention to provide an improved non-reinforced polyoxymethylene composition for an injection molding process.
Another object of the present invention is providing a process for the preparation of a stabilized non-reinforced polyoxymethylene molding composition employing a specifically defined class of polycarbodiimides not employed heretofore in the art with respect to the stabilization of polyoxymethylenes.
These and other objects of the invention will become apparent from the following summary and description of preferred embodiments.